1. Field of the Invention
The present invention relates to polycrystalline semiconductor processing and, more particularly, is directed towards a method for forming thin large-grain polycrystalline films from amorphous films for use as generally large-grain solid state devices where crystallographic orientation is not important, such as, for example, photovoltaic devices. The resultant polycrystalline film is characterized by a grain size that is greater than the thickness of the film.
2. The Prior Art
Typically, thin large-grain polycrystalline films are conventionally grown in suitable furnaces by long, high temperature annealing cycles of thin small-grain polycrystalline films. In these processes, the driving force for grain growth is the reduction in grain boundary surface energy per unit volume with increased grain size. This driving force decreases inversely with grain size but becomes even smaller once the grain size approaches or exceeds the thickness of the film. Furthermore, at this stage of grain growth, grain boundary velocity and/or mobility also decrease due to interactions between the grain boundaries and the surface of the film. The lowered driving force for grain growth and the reduced grain boundary velocity and/or mobility combine to set an effective limit on the ultimate achievable grain size and that is approximately equal to the thickness of the film. Further grain growth occurs only with excessively long annealing times at very high temperatures, which require prohibitively high energy consumption levels, a procedure that can also introduce junction diffusion and contamination from the substrate or at the surface.
Another present day method for improving the crystallinity of semiconductor films involves the use of a laser beam matched to the film. By scanning the surface of the film, the matched beam delivers sufficient energy thereto to create a melt in the film and crystallization occurs by propagating the melt in the film along the scan track. U.S. Pat. No. 4,059,461. A further present day method also converts a small-grain polycrystalline film into large-grain polycrystalline or single-crystal film by crystallizing from a localized melt. U.S. Pat. No. 4,152,535. A still further method heats a substrate by direct current passsage in a vapor-phase deposition process in which 80 to 100% of the deposited film is subsequently melted from the free surface downward and resolidified from the substrate body upward. U.S. Pat. No. 4,113,532.
These present day methods of melting and reforming small-grain polycrystalline or amorphous films into single-crystal or large-grain polycrystalline films are economically handicapped, and remain so handicapped, because of the low values for maximum grain growth rates which are intrinsic to such growth processes, high energy investment and/or excessive contamination especially for processes which create melts.